The invention is directed to a process for manufacturing a housing for sensor elements with at least one outer housing element comprising plastic material. The invention is also directed to a sensor and its use as a temperature sensor.
In the following, plastic components with pliable or rubber-like properties, for example, thermoplastic elastomers or elastics, or TPE, are designated as soft plastic components, while plastic components which no longer have practically any deformability after manufacture, for example, polyamide or PPS-are described as hard, optionally even as hardenable, plastic components or as hard material of a plastic injection encapsulation.
From German published patent application DE 197 57 006 A1, a process is known for manufacturing measurement value receivers having a sensor element and a housing comprising a plastic material, wherein a so-called xe2x80x9cMolded Interconnected Device (MID)xe2x80x9d manufacturing process is used. Here, in a first manufacturing step of the MID process, a carrier element filled with palladium is first injection molded. In a second operation, the areas of the carrier element on which no electrically conducting elements are to arise are coated with a plastic protective layer. In a third process step, the electrical connection leads are then galvanically applied on the non-encapsulated surface of the carrier element, whereby the conductor paths are formed. In a fourth operation, the sensor element and the associated measuring circuit are applied. The resulting mounted components in the area of the sensor element still covered with a plastic or ceramic top can be cemented or heat-caulked. Subsequently, in a fifth and final process step, the entire arrangement is injection-encapsulated with plastic. This involves a comparatively expensive manufacturing process.
Furthermore, from European patent EP 0 539 555 B1, a housing structure around an operating device is known wherein, as the operating device, for example, a semiconductor element, an element based on a semiconductor, a sensor element, a microactuator, or an electronic circuit comprising one or more integrated circuits comes into question. The housing forms at least one closed off hollow space, which surrounds the operating device either wholly or partially, whereby metal parts run through the walls of the housing and form wire connections to the operating device surrounded by the housing. The hollow space is thereby sealed off and filled with an electrically insulating fluid. The housing is made of a plastic molding compound, wherein the plastic molding compound is a thermoplastic, and at least parts of the walls surrounding the operating device are designed to be pliable, in order to accommodate expansion forces. For the housing structure, the surrounding housing around such an operating device is formed by two or more components. During assembly at least one hollow space results. This involves a comparatively complex construction, whose realization is associated with comparatively high costs.
Furthermore, from German patent DE 196 21 000 C2, a temperature sensor is known, which has a measuring resistor comprising a substrate with an electrically insulating surface and a resistance layer situated on it and surrounded by a casing of temperature-resistant plastic as well as an additional outer housing of plastic, through which the connection leads of the measuring resistor are passed. The casing serves as a primary housing which, together with the embedded measuring resistor in the form of a resistance layer, is surrounded by a plastic injection-encapsulation as an outer housing. Here the connection leads are passed outward through the plastic encapsulation, the measuring resistor is constructed as an SMD component and is sealed against its environment by means of the plastic casing. Consequently, at least two of the connection leads between the measuring resistor and the outer connection contacts are passed gas- and liquid-tight though the plastic casing and the plastic injection encapsulation, so that the sensor is also sealed off against the ambient atmosphere.
It has proven to be problematic that, upon solidification of the plastic injection encapsulation, the measuring resistor can be impaired in its accuracy due to shrinkage processes.
An objective of the invention is to provide a process for the direct housing of sensors, particularly temperature sensors based on resistors, wherein a high accuracy of the sensor element is maintained. Furthermore, a sensor manufactured according to the processes should be provided.
The objective is accomplished in accordance with the invention in that the housing is manufactured in a two-component injection molding process, wherein in a first process step the sensor is injection-encapsulated by a soft plastic component and in a second process step, a second plastic component is applied, which forms the outer housing for the sensor element as a hard plastic component.
It proves to be particularly advantageous that, due to the soft component, no shrinkage processes affect the sensor element and thereby its accuracy. A further advantage is to be seen in that customer-specific solutions-for example special plugs-can be applied at the electrical connection of the sensor.
In a preferred embodiment of the process, the sensor element is provided with at least two connection leads, which are at least partially encapsulated by the soft plastic component. Advantageously, the connection leads of the sensor element, together with connection points for an external supply leads, are injection-encapsulated by the soft plastic component. In practice, in a first process step, the sensor element is encapsulated by an elastomer or an elastic.
The objective is accomplished with respect to device aspects for a sensor, especially a temperature sensor, having a sensor element which is enclosed by a housing having at least one plastic component, in that the sensor element is encased by a soft plastic component, which in turn is arranged in a plastic component serving as an outer housing, as an encapsulation made of a hard material.
The objective is further accomplished for a sensor, especially for a temperature sensor, having a measuring resistor as a sensor element on a substrate with an electrically insulating surface and a resistance layer situated on it, wherein the measuring resistor is enclosed by a housing made of hard plastic molding compound, in that the measuring resistor is encased by a soft plastic component, which in turn is arranged in a plastic component serving as an outer housing, as an encapsulation made of a hard material.
In a preferred embodiment of the sensor, the measuring resistor is connected via at least two connection leads to an evaluation circuit connectable from outside, wherein the connection leads are at least partially encased by the soft plastic component. Here, the connection leads are advantageously passed through the soft, as well as through the hardenable plastic component.
For customer-specific uses, at least one end of a connection lead is mechanically and electrically connected with one end of a plug.